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Alexander Huth / The Regents of the University of California By Aviva Rutkin Most English dictionaries list words alphabetically, but how do we store them in our head? Finding out could have an unexpected pay-off: being able to tell what someone is thinking from their brain activity. Although neuroscientists can already do this to a limited extent, the brain’s internal filing system for words and concepts – an important step towards accurately reading a person’s thoughts – remains murky. Now Jack Gallant at the University of California, Berkeley, and his team have charted the “semantic system” of the human brain. The resulting map reveals that we organise words according to their deeper meaning, in subcategories based around numbers, places, and other common themes. Previous “mind-reading” studies have shown that certain parts of the brain respond to particular words. Gallant’s own lab had already found that the brain sorts visual information by meaningful categories like animals or buildings. In their latest experiment, the team wanted to see if they could build a more complete map of meaning across the cerebral cortex, the folded outer layer of grey matter. To do this, they asked seven people to listen to two hours of The Moth Radio Hour, a show which features individuals telling stories. As this happened, they used an fMRI scanner to log changes in blood oxygen levels across the brain – a sign of neural activity. The team then compared the meanings of the words in the show against the activity in small subregions of the brain. They identified 12 categories of words – concepts such as time, location, emotion or social relevance – that seemed to activate more than 100 brain regions in different ways. Other categories included visual words (for example “yellow”), work concepts (“meetings”), tactile words (“fingers”), and abstract ideas (“nature”). The team then used software to plot clustering data from six people on a single brain map, pictured below. It charts a complex pattern of activity across more than 100 areas spanning both brain hemispheres. This is a surprise as the brain’s left side is generally considered to be responsible for language. Alexander Huth / The Regents of the University of California The map suggests that patterns of word meaning are consistent between different people’s brains, but the team say this might be because they studied a small number of people with a culturally similar upbringing and education. With a map like this, the team also suggest it may be possible to build a “general-purpose language decoder”, a device that can infer what someone hears or says using fMRI data alone. Scientists have long suspected that words are organised into clouds of meaning in the brain, says Richard Wise, a neurologist at Imperial College London. “The results won’t really surprise anybody,” he says. But by studying people while they listened to stories, rather than isolated words or sentences, the team has assembled a useful picture of how the brain responds to the kind of language we hear every day, says Swathi Kiran of Boston University. “They’ve essentially put it all together.” The map may help us understand language deficits in Alzheimer’s disease or in aphasia, a condition which can involve using the wrong words or sounds in speech. “We’re struggling with brains that are not this nice and neat and have got damage,” says Kiran. “This paper tells us what normal could look like.” To explore the map, check out the team’s interactive version. Journal reference: Nature, DOI: 10.1038/nature17637 Read more: Brain decoder eavesdrops on your inner voice; Ancient whistle language uses whole brain More on these topics: